1. Field of the Invention
This invention relates to an image processing system and an image processing system suitably used for a digital copying machine, a digital image recording apparatus or the like.
2. Description of the Related Art
Conventionally, a device for conversion into multi-value to convert a low-tone image (i.e., N-valued image) into a high-tone image (i.e., M-valued image, N&lt;M) has been well known. For example, there has been provided a device for converting a binary image into a multi-valued image.
In case of varying the magnification (conversion of resolution) of an N-valued image, the N-valued image has been once converted into a multi-valued image using the device for conversion into multi-value. In addition to the above case, the device for conversion into multi-value has been used when a binary image is converted into a multi-valued image of high quality for outputting like a case of FAX, or when the binary image is compressed with binary data for storing, and then restored and converted into a multi-valued image when outputting.
Various techniques for conversion into multi-value have been developed heretofore and can be generally classified into four groups as follows.
(1) Method of using a simple smoothing process for conversion into multi-value (See Unexamined Japanese Patent Publication (kokai) Nos. 63-013579, and 5-160996 or the like)
(2) Method of converting an image into a binary image using a systematic dither process, and subsequently estimating an original intermediate-tone image (i.e., multi-valued image) in the binary image using dither matrix having been used at the time of conversion into the binary image (Unexamined Japanese Patent Publication (kokai) No. 61-288567 or the like)
(3) Method of detecting brightness (i.e., number of black picture elements) within a certain area and then appropriately switching over the size of a filter for conversion into multi-value according to the detected brightness (Unexamined Japanese Patent Publication (kokai) No. 2-76370 or the like)
(4) Method of detecting intensity and direction of an edge and then appropriately switching over the size, shape or coefficient of a filter for conversion into multi-value according to the detected intensity and direction (Unexamined Japanese Patent Publication Nos. 5-143726 and 5-344340 or the like)
According to the above conventional art, an area of a predetermined size containing a specific picture element (i.e., a candidate picture element for processing) has been set for an input image of low tone. Subsequently, an image of high tone has been estimated according to a picture element value in the predetermined area.
However, in the above conventional art, a device for conversion into multi-value has been used without variation, irrespective of the resolution of an input image. Thus, inappropriate conversion into multi-value has been caused in some cases.